It is important in many applications to establish a dependable electrical ground connection. Grounding requirements may necessitate that particular electrical equipment, or one side of an electrical circuit, be maintained at zero potential difference with respect to absolute ground potential, namely, the earth. For example, one side of a typical single-phase electrical distribution circuit typically is maintained at ground potential. One simple way to provide an earth ground connection is to drive a metal stake or grounding rod into the earth and extend a wire or other suitable conductor from that grounding rod to the apparatus or circuit to be maintained at ground potential.
The use of alternatives to an electrical grounding rod soon evolved for establishing or maintaining electrical grounding in practical applications. These alternatives typically included utility lines, such as water pipes, buried in the earth. Such utility pipes typically utilized electrically-conductive metals such as iron or copper pipe, and are in intimate contact with the earth before entering a building. Accordingly, a suitable grounding connection could be made to a water pipe, for example, within a particular structure, with knowledge that the ground connection thus made would be at earth ground potential extending along the electrically-conductive pipe system to a point where the pipe entered into contact with the earth.
In more recent times however, pipes or conduits made of electrical non-conductors such as PVC have been substituted for copper or iron pipe and other plumbing fittings, as such non-metallic pipe and fittings are less expensive to manufacture and install into a plumbing system and are not subject to corrosion, i.e., when buried in the earth. PVC pipe is especially in widespread use for cold-water lines that otherwise would provide a direct path to earth. Moreover, water lines extending from dwellings to an upstream water main, and indeed the water mains themselves, increasingly are made of PVC or another non-metallic material. A conventional plumbing system having even a single portion of non-metallic material cannot provide a ground connection, when a person connects a ground wire to a metallic plumbing component downstream in the plumbing system from the non-metallic element. As a result, the risk exists that someone may believe an electrical component or circuit is at ground after making a connection to a metallic plumbing element, such as a localized metal pipe or faucet within the plumbing system, when in fact the presence of an intervening non-conductive pipe or other plumbing element prevents electrically-conductive path extending from that element to an earth ground.
Fuel delivery systems are another application where proper grounding is essential to reduce the risk of fire or explosion. Fuel supply pipes leading to fuel delivery pumps from underground fuel storage tanks, as well as the tanks themselves, increasingly are made of nonmetallic material that does not provide a ground path from the pump to earth. If a vehicle operator acquires a static-electricity charge, e.g., sliding across certain kinds of seat upholstery when exiting the vehicle at a fueling station, that static charge may transfer to the metallic fuel delivery nozzle of a gasoline pump. The static charge on the nozzle may cause a spark to jump from the nozzle to the fuel filler pipe of the vehicle as the nozzle is moved into contact with the fuel filler pipe. If a sufficient concentration of gasoline vapor is present at the entrance to the filler pipe, that spark can ignite a fire.